MTEL Chemistry common mistakes
MTEL Chemistry · Understand the traps and how to avoid them.
Balancing a redox equation by inspection and stopping once atom counts match, without checking that total charge and electron transfer also balance.
Candidates learn atom-counting in introductory balancing and carry that habit into redox items; they treat charge as a cosmetic label rather than a conserved quantity, and they don't separate the oxidation and reduction half-reactions.
What to do instead
For every redox item, write the two half-reactions, balance O with H2O and H with H+, add electrons to equalize charge, then combine and cancel. Verify that the sum of oxidation-number changes is zero and that net charge is identical on both sides before selecting an answer.
Predicting products of a single- or double-replacement reaction from element identity alone, ignoring solubility rules and the activity series.
Candidates memorize a handful of reaction 'types' and pattern-match the reactants, so they write a precipitate or a displacement that thermodynamics does not allow; they also forget that a reaction may be 'no reaction'.
What to do instead
Before writing products, apply the solubility table to the hypothetical precipitate and the activity series to the hypothetical displaced metal. If the putative product is soluble or the metal is less active, conclude no reaction. Practice by generating the ion-swap product first, then testing it against the rules.
Computing pH from a weak-acid concentration by taking -log[HA] directly, as if the acid dissociated completely.
The strong-acid formula is the first one learned and is reinforced by many practice items; candidates under-weight the Ka information or don't recognize that Ka < 1 signals partial ionization.
What to do instead
Set up an ICE table with Ka, assume x << [HA] only after confirming that [HA]/Ka > 400 (or solve the quadratic otherwise), then take -log x. Always check that the calculated percent ionization is small; if it is not, redo without the approximation.
Choosing the answer with the largest or smallest number in a stoichiometry item without converting all quantities to moles and identifying the limiting reagent.
Mass, volume, and molarity are given side by side, and candidates compare the raw numbers because the arithmetic feels faster under time pressure; molar mass differences make the raw comparison meaningless.
What to do instead
Adopt a fixed three-step routine for every stoichiometry item: convert each reactant to moles, divide by its coefficient to find the limiting reagent, then convert the target amount back to the requested unit. Do the mole conversion even when one reactant looks obviously scarce.
Treating a system at equilibrium as static, so a change in conditions is predicted to shift the reaction 'to completion' rather than to a new equilibrium position.
Le Chatelier's principle is often memorized as a set of directional arrows without the underlying idea that Q must return to K; candidates also confuse a shift in position with a change in the value of K.
What to do instead
For each disturbance, write Q before and after the change and compare it to K. State explicitly whether K itself changed (only temperature does that) and describe the new position qualitatively. Rehearse with pressure changes on reactions having equal moles of gas on both sides, where there is no shift.
Assigning molecular geometry from the number of electron domains without first distinguishing bonding domains from lone pairs, so bent and trigonal pyramidal molecules are mislabeled as linear or trigonal planar.
VSEPR is taught as a table keyed to steric number, and candidates stop at the electron-domain geometry; lone pairs are drawn but not subtracted when naming the molecular shape.
What to do instead
Draw the Lewis structure, count bonding domains and lone pairs separately, assign the electron-domain geometry, then remove lone pairs from the description to get the molecular geometry. Memorize the specific exceptions (water bent, ammonia trigonal pyramidal, sulfur dioxide bent) and predict bond angles as 'less than' the parent geometry.
Balancing a nuclear equation by matching mass numbers only, or by treating beta-minus emission as a loss of a proton.
Mass and atomic number are two separate conservation constraints, and the beta particle's identity (an electron emitted when a neutron converts to a proton) is counterintuitive because the atomic number rises while a negative particle leaves.
What to do instead
For every nuclear item, write both conservation equations: sum of mass numbers equal, sum of atomic numbers equal. Then solve for the unknown. Drill the four common modes (alpha, beta-minus, beta-plus/positron, electron capture) by writing their symbols and the resulting change in Z and A before touching the problem.
Reading a heating curve or phase diagram and reporting a temperature change during a phase transition, or using q = mcΔT for the plateau regions.
The single specific-heat formula is over-practiced, and candidates apply it uniformly; they also misread which axis is pressure versus temperature on a phase diagram and misplace the triple point.
What to do instead
Segment every heating-curve calculation into sloped regions (q = mcΔT) and flat regions (q = nΔHfus or nΔHvap), and sum them. On phase diagrams, locate the triple point and critical point first, then trace a vertical line at constant composition to see which phase changes occur.
Confusing the signs of ΔH and ΔS when judging spontaneity, or assuming a negative ΔH alone guarantees a spontaneous reaction.
Exothermic reactions are described as 'favorable' in everyday language, and candidates collapse two independent driving forces into one; they also forget that ΔG = ΔH - TΔS makes the answer temperature-dependent.
What to do instead
Build the four-case table (ΔH negative/positive crossed with ΔS negative/positive) and state for each whether the reaction is spontaneous at low T, high T, all T, or never. Then use ΔG = ΔH - TΔS to find the crossover temperature when the signs oppose.
On laboratory-safety and measurement items, choosing the procedure that gives the most precise-looking number rather than the one that is safe and metrologically correct, such as reading a burette from the top or using a graduated cylinder for precise volumes.
Candidates focus on calculation and treat glassware choice and safety as incidental; they also don't connect significant figures to the precision of the instrument actually used.
What to do instead
Learn the precision hierarchy (volumetric flask and burette > graduated cylinder > beaker) and match each instrument to its purpose. For safety items, eliminate any option involving tasting, direct smelling, or heating a closed system first, then select among the remaining procedures by standard practice.